There's nothing wrong with wanting to save a little money. There are plenty of legitimate coupon sites out there but here's the skinny on coupon sites:
If they want you to install software to get a coupon: Don't.
It's that simple. It's one of those "no good can come of this" things. It will install some junk toolbar or addon into your browser, slowing it down. You'll get popups on webpages or your webpages will suddenly be filled with ads in places where there wouldn't be normally. It's just not worth it.
If you find a coupon site that gives you a simple discount code. Bookmark it and stick to it.
If a site says "download this to get coupons", close that tab and find a different site.
More info at Malwarebytes' blog